Glass glazing units are commonly used to glaze openings in buildings, but such units must be constructed with the utmost of care to prevent condensation build up, where its presence is viewed as detrimental to the functioning of the glazing system, for reason of aesthetics, grow of mold and algae, and interference with view and light transmittance.
Before the widespread introduction of sealed insulated glass units, most windows were single glazed. In cold climates, it was common to have a removable ‘storm window’, or second glazing layer in a wood framing system, that was installed over the outside of a single glazed window in the fall, and removed in the spring. The air gap between the two glazings was vented to the exterior (the cold side) through holes in the lower front face of the window. It was typical to use several holes, each an inch or more in diameter. By connecting the air gap to the outside where air is colder and of lower absolute humidity, condensation on the inside surface of the cold outer glass was minimized.
The industry has since moved to sealed glass glazing units. The conventional insulated glass unit is constructed of two lights of glass, surrounded by a spacer which is typically filled with a desiccant, and covered by an organic sealant with a low moisture vapor diffusion transmittance. The inside edge of the spacer is perforated so that the desiccant within dries the air in the interior of the unit, preventing condensation. There is a low but finite diffusion of moisture through the sealant, and typically these systems last for two or more decades before the appearance of internal condensation. However, should the seal fail (typically it is the bond between the glass and sealant that fails) then the unit ‘breaths’ exterior air as a result of natural pressure changes that are induced by daily temperature changes, and the moisture brought within quickly saturates the desiccant resulting in internal condensation, and this defines failure of the unit.
As known in the industry, there are two main types of glass units used in glazing units: vision (transparent) and translucent.
Vision glass glazing units have problems unique unto themselves. Vision glass units contain desiccant to avoid condensation in the cavity between the glass lites. For instance, in mountainous regions, if the difference in altitude between the place where an insulated glass unit is manufactured and the place where it is installed, is large enough, then the pressure differential could stress or damage seals, or break the glass, typically during transportation. In order to deal with this, capillary tubes are sometimes used to provide temporary pressure equalization for insulated glass units. The standard in North America is a stainless steel tube, 12″ long and 0.020″ inside diameter, installed through the perimeter seal in order to provide a controlled flow path between interior and exterior. The units are shipped to site, and allowed to pressure-equilibrate for a day or more, and the tubes are then crimped to return the unit to a fully sealed state. Although it may be advantageous for certain reasons to leave the tube open (lower stress on glass and seals as pressure differential varies), technical literature teaches that if left open, the airflow will carry in sufficient moisture to saturate the desiccant in a relatively short period of time, resulting in moisture-related failure of the unit. This ingress of moisture will fill the desiccant with moisture much sooner than if the unit was fully sealed.
Venting a vision glass unit without a desiccant is not an option as under certain conditions then some condensation will occur. Vision glass units are especially susceptible to condensation because the glass lites are transparent and condensation is very obvious.
If an ordinary vision glass unit containing desiccant within the internal airspace is vented, the desiccant will keep the air inside the unit drier, until is absorbs to capacity. Since common desiccants are two-way, the desiccant now acts as a moisture sink. In other words, the desiccant acts to equilibrate the unit to a relative humidity, so if a lite gets cold (for example during a winter evening), water will condense on the lite. This reduces the RH in the unit which in turn pulls more moisture from the desiccant. The unit then further condenses, resulting in a really wet-looking unit.
Any glass unit that is sealed will undergo internal pressure change in glass and ‘pillow’ in response. The amount of movement of the glass required to relieve the pressure change is directly proportional to thickness of the unit. Length or width of the unit is of secondary importance. In day to day conditions, glass can easily accommodate this pressure change if the spacing between lites is 0.5″. At 1″ or larger, glass breakage occurs because there is a larger quantity of air in the unit which expands when heated, and even if breakage does not occur, there is a resultant increase in stress on the seals which will lead to higher incidence of seal failure.
The Applicant is aware of a company called VisionWall that makes a large thickness vision glass unit with somewhere around 2.5″ of gap. These units must be vented to relieve pressure differentials, and this is done so through tubing and a large replaceable vent cartridge that is full of desiccant (rather than having the desiccant in the glazing cavity as in normal units). This keeps inward flowing air dry until, after about a decade, the desiccant reaches capacity and must be replaced as part of regular maintenance.
A difference between vision glass glazing units and translucent glass glazing units is that translucent glazings contain absorptive material filler in the gap or cavity between the lites. This alters the physics of the system and allows different approaches to condensation control. Translucent units are typically thicker to accommodate the presence of this filler and it is understood in the industry that the larger gap can introduce a requirement for venting.
Referring to FIG. 1, the Solera™ translucent glazing unit (manufactured by the present applicant) has typically 2.5″ gap between two lites of glass 100a and 100b and maintained by spacer 110, which is 5 time industry standard. The gap contains absorptive filler 130. These units will often break if fully sealed by sealant 120. It is generally accepted in the industry that gaps less than 1″ is ‘normal and sealable’, while a gap of over 1″ is considered thick and in need of venting.
Another difference between vision glass glazing units and translucent glass glazing units is that the absorptive filler in the cavity will absorb some energy from sunlight, thus increasing temperature cycling and increasing the need for venting. The absorptive materials also adsorb and desorb moisture as temperature and humidity change.
The standard approach to address the problems associated with translucent glass glazing units is to build units with gaps that are typically on the order of 0.5″, fully seal the unit, and include sufficient desiccant within the gap on the perimeter to draw most of the moisture out of the initial ‘air fill’, with excess capacity to soak up the moisture that slowly but inevitably diffuses through the polymer seals. However, this solution has a number of drawbacks. First, the units are restricted mostly gaps less than 1. Second, the life of the unit is finite as diffusion will occur. Third, this construction is extremely sensitive to the slightest seal failure. For example, a slight contamination from, say, a fingerprint on the glass where the sealant contacts it can cause failure. Fourth, visible ‘pillowing distortion’ can be observed on reflective coated units at certain times of day which ruins aesthetics for some architects. Fifth, glass stress is increased, increasing statistical occurrence of breakage.
Referring again to FIG. 1, typically in translucent glass glazing units, venting is done by connecting the interior of the spacer to the intraframe cavity via a capillary tube 140. But this system had little or no ability to maintain a humidity differential such that the interior was dryer on average than the intraframe cavity. The result is that condensation can occur in transient conditions (usually upon a sudden change in weather with rapid cooling), which is not considered acceptable in the industry.